If you have a keen eye, you probably noticed the 1-pixel black bar at the top of your screen when you enter F11-fullscreen in Firefox.
To fix this, you'll need to create this file:
/* …/Profiles/xxxxxxxx.default-esr/chrome/userChrome.css */
#fullscr-toggler {
// background-color: red;
display: none;
}
(If you can't figure out how to open that file or create it in the first place, this tutorial seems pretty good for Windows. On Linux, it's hiding inside ~/.mozilla/firefox, which is a hidden folder you can probably access by opening your file manager and pressing Ctrl–H. Note that you will likely have to create the subfolder chrome.)
Once that CSS file exists, enable toolkit.legacyUserProfileCustomizations.stylesheets in about:config, and restart the browser.
